(I) Field of the Invention
The present invention relates generally to gas separation membranes cast from a miscible two-component or three-component polymer blend of (a) aromatic polyethersulfones with (b) an aromatic polyimide I, and optionally (c) an aromatic polyimide II, polyamide or polyamide-imide. The blends provide gas separation membranes cast therefrom. The precise gas transport properties of the membrane may be adjusted based on the blending of the specific polymer components.
(II) Description of Related Art
It is well known to modify polymer properties through blending to obtain inexpensive materials with improved characteristics. Polymer blends are broadly classified into homogeneous (miscible) and heterogeneous (immiscible) blends. Homogeneous blends are relatively rare; however, a homogeneous blend of an aromatic polyethersulfone and an aromatic polyimide has been described in the art. In particular, "Thermal and Rheological Properties of Miscible Polyethersulfone/Polyimide Blends" by K. Liang, J. Grebowicz, E. Valles, F. E. Karasz and W. J. MacKnight, published in the Journal of Polymer Science: Part B: Polymer Physics, Vol. 30, pp. .sup.465-476 (1992) describes a miscible blend of an aromatic polyethersulfone (commercial name VICTREX.RTM.) and an aromatic polyimide (commercial name MATRIMID.RTM. 5218) over the entire blend composition range, particularly blends containing less than 10% of the aromatic polyimide.
Aromatic polyethersulfones are not, however, miscible with other aromatic polyimides, polyamides or polyamide-imides. This immiscibility, along with excellent high-temperature properties and high softening temperatures, have made blends of aromatic polyethersulfones and aromatic polyimides, polyamides or polyamide-imides very difficult to fabricate into many products. The fabrication of films, coatings, and other structures from blends of aromatic polyethersulfones and aromatic polyimides, polyamides or polyamideimides generally requires elaborate processes. Particularly, it is known that the aromatic polyethersulfone having the following repeating formula: ##STR1## is not miscible with certain alkyl substituted aromatic polyimides, polyamides or polyamide-imides.
It is also generally known that gas separation membranes may be cast from polymers. The separation of gas components by polymer membranes is thought to depend on chemical affinities, kinetics and structural characteristics; it being known that rubbery polymers are generally characterized by high diffusion and relatively low selectivity while glassy polymers are generally characterized by lower diffusion and higher selectivities.
Polymer blending has traditionally been- thought to be either problematic or of no benefit in the membrane field, primarily because different polymers are generally not miscible with one another. Those few polymers which are thought to be miscible offer no blending advantage in the membrane field because of various reasons, including difficulty in blending, poor mechanical properties, limited range of gas transport properties, and complex relationships between blend composition and gas transport properties.
U.S. Pat. No. 5,055,116 describes a blend of certain aromatic polyimides, in which the blending of the polymer components is adjusted to achieve certain permeability and selectivity of a polymer membrane. The final properties of a new polymer membrane may be predicted so that a membrane with those desired final properties can then be manufactured.